Radio communication systems, including two-way communication systems, are well known in the art. In many such systems, the communication channel (wherein the "channel" may be a dedicated frequency, a frequency or frequency pair allocated pursuant to a frequency division multiplexing scheme, one or more assigned time slots in a time division multiplex system, a code division multiplexed channel, or other radio frequency communication path) must first be allocated or otherwise established prior to or co-extensive with initial usage, and knocked down or otherwise made available following usage. In many such systems, the knock down mechanism, in addition to relinquishing the radio link, also facilitates avoidance of squelch tail audibilization at receiving radios.
In analog clear voice radio systems, carrier squelch detects, analog tones, or digital codes are utilized to avoid squelch tail noise. Similarly, digital codes serve this same function in many digitally encrypted systems. In all of these cases, upon detecting the appropriate signal, the receiving radio sequelches its audio processing, thereby avoiding or minimizing audibilization of a squelch tail.
The above described methodologies work well enough in an appropriate application. Where, however, a particular application requires the use of multiple packets of encoded voice information (such as CELP or VSELP encoded voice), such prior art approaches are not necessarily sufficient. This becomes particularly true as throughput demands increase on the one hand, and the need for rapid channel set up and knock down increases on the other.
To illustrate at least one aspect of the problem in more detail, consider the following example. In a typical voice encoded communication system, an initial voice message is appropriately processed into a corresponding plurality of information packets, wherein each packet contains digital information representing an encoded representation of a portion of the original voice message. These packets are then transmitted in serial fashion, along with some system signalling information.
The system signalling information may include, for example, an end-of-message (EOM) indicator or the like. Upon detecting an EOM at the conclusion of a message, a receiver can appropriately squelch further audio processing, in accordance with prior art technique as already noted above. Typically, however, such EOMs comprise a relatively small amount of information relative to the full message, and in a multipath fading environment (such as that presented in land mobile radio), such signalling information may well be lost from time to time.
To accommodate such a fading environment, many receivers in such a system will compensate for the lack of currently available viable packets by substituting previously received good packets. This substitution then continues until either valid information again reappears, or a time-out sequence concludes.
This arrangement functions well to assist the receiver during short fades that occur while receiving encoded voice packets. When, however, a fade coincides with the EOM signal at the conclusion of a message, considerable mischief results. In particular, the receiver, unaware that the link has been brought down, continues to monitor for additional valid voice packets. In the absence of receiving such packets, the receiver reprocesses previously received voice information, and this reprocessing continues until a time-out sequence concludes. This reprocessed information, rendered audible by the receiver during the time-out period, constitutes an objectionable squelch tail-like artifact.
One way to avoid the above problem, or to at least minimize it, would be to include significantly more control signalling into the information stream. Inclusion of such additional signalling information would render less likely a devastating coincidence between a debilitating fade and an EOM indicator. Increasing overhead signalling, however, will typically require a commensurate reduction in throughput capability for voice information, and this will usually result in a concurrent loss of voice quality.
Accordingly, a need exists for an end-of-message methodology that will not increase overhead signalling while simultaneously minimizing squelch tails and related phenomena during end-of-message processing.